1. Field of the Invention
The present invention relates to an optical pickup apparatus and a disc apparatus including the same.
2. Description of the Related Art
An optical disc is inserted into an optical disc device (not shown) provided with an optical pickup apparatus. The optical disc to be inserted into the optical disc device, not shown, is formed in a substantially disc shape.
The discs include data read-only optical discs such as “CD-ROM”, “DVD-ROM”, “HD DVDROM” and “BD-ROM”, data recordable optical discs such as “CD-R”, “DVD-R”, “DVD+R”, “HD DVD-R” and “BD-R”, data writable/erasable and data rewritable-type optical discs such as “CD-RW”, “DVD-RW”, “DVD+RW”, “DVDRAM”, “HD DVD-RW”, “HD DVD-RAM” and “BDRE”, and the like.
Explaining about the optical discs, “CD” is an abbreviation for “Compact Disc” (trademark). “DVD” (registered trademark) is an abbreviation for “Digital Versatile Disc”. “HD DVD” (registered trademark) is an abbreviation for “High Definition DVD”. “Blu-ray” as used in “Blu-ray Disc” (registered trademark) refers to a blue-violet laser beam adopted to realize a higher density recording than a red laser beam. “HD DVD” is compatible with the DVDs and also has a larger storage capacity than those of the DVD discs. For the CDs, an infrared laser beam is used. For the DVDs, a red laser beam is used. Whereas, when reading out data/information/signal stored on optical discs such as the “Blu-ray Disc” or “HD DVD” or when writing data/information/signal on optical discs such as “Blu-ray Disc” or “HD DVD”, a blue-violet laser beam is used.
“ROM” as used in “CD-ROM”, “DVD-ROM” and “HD DVD-ROM” is an abbreviation for “Read Only Memory”. “BD-ROM” is an abbreviation for “Blu-ray Disc-ROM”. “CD-ROM”, “DVD-ROM”, “HD DVD-ROM” and “BD-ROM” are those of a data/information read-only type. “R” as used in “CD-R”, “DVD-R”, “DVD+R”, and “HD DVD-R” is an abbreviation for “Recordable”. “BD-R” is an abbreviation for “Blu-ray Disc-R”. “CD-R”, “DVD-R”, “DVD+R”, “HD DVD-R” and “BD-R” are those of a data/information writable type. “RW” as used in “CD-RW”, “DVD-RW”, “DVD+RW” and “HD DVD-RW” is an abbreviation of “Re-Writable”. “BD-RE” is an abbreviation of “Blu-ray Disc-RE”. “CD-RW”, “DVD-RW”, “DVD+RW”, “HD DVD-RW” and “BD-RE” are those of a data/information rewritable type. “RAM” as used in “DVD-RAM” and “HD DVD-RAM” is an abbreviation of “Random Access Memory”. “DVD-RAM” and “HD DVD-RAM” are data/information readable/writable/erasable.
An optical disc on which data/information/signal can be recorded in an optical disc device includes a signal layer serving as a signal recording surface of the optical disc and being provided with a groove (not shown) in which the data/information/signal is to be stored. The groove means an elongated recess, for example. When a disc-shaped optical disc is seen in a plan view, the groove is formed in a substantially helical shape. If the optical disc is seen from a signal layer side which, when the optical disc is irradiated with a laser beam, is a side irradiated with the laser beam, the groove is spiral. Since the groove is extremely small, the groove cannot be visually recognized.
The optical pickup apparatus is provided with an optical system that detects error signals such as a focus error signal and a tracking error signal so as to appropriately form a light collection spot to a predetermined recording track on the optical disc by controlling the position of an objective lens.
A focus means a focal point, for example. “Focusing” means to focus on a point or to bring into focus. “Tracking” means to track and observe, for example, using light, a signal layer of an optical disc or a very small pit (hole, dent), a groove, a wobble and the like provided in the signal layer of the optical disc and to determine a position of a track drawn in a substantially spiral shape. “Pit” means a hole or a recess, for example. “Wobble” means wobbling of a track in which a data signal such as information is recorded, for example.
Methods of detecting focusing of a light collection spot on an optical disc in an optical pickup apparatus include a detecting method based on, for example, a differential astigmatism method. The differential astigmatism method is a method of detecting displacement of a light collection spot by detecting a point-image distortion formed by an optical system with astigmatism, for example. Further, methods of detecting tracking of a light collection spot on an optical disc in an optical pickup apparatus include a detecting method based on, for example, a differential push-pull method. The differential push-pull method is, for example, a method of detecting displacement of a light collection spot by means of a main beam for data reading/writing and two sub-beams for detecting a correction signal of deviation.
Describing the method of detecting a tracking error signal, in a case where a tracking error signal is detected by an optical pickup apparatus for an optical disc conforming to the CD standard (CD-ROM, CDR, CD-RW and the like) having a track pitch of 1.6 μm (micron/micrometer), for example, a “three-beam method (or also referred to as a three-spot method) in which three light fluxes are used is mainly adopted as a method of detecting a tracking error signal. Also, if a tracking error signal is to be detected by an optical pickup apparatus for an optical disc conforming to the DVD standard (DVD-ROM, DVD-R, DVD-RW and the like) having a track pitch of 0.74 μm, for example, an “inline method” in which at least three light fluxes are used, for example, is mainly used as a method of detecting a tracking error signal. The name of each of the methods of detecting a tracking error signal as used here is a name provided for the sake of convenience.
A track pitch of a DVD-RAM of Version 1 having a land/groove structure is approximately 0.74 μm, for example, while a track pitch of a DVD-RAM of Version 2.0 and Version 2.1 having a land/groove structure is approximately 0.615 μm. Also, for example, a track pitch of the DVD-ROM, DVD-R, DVD-RW and the like having a structure different from the land/groove structure is approximately 0.74 μm, while the track pitch of the DVDRAM of Version 2.0 and Version 2.1 having a land/groove structure is approximately 0.615 μm. As described above, the track pitch is different for the DVD-RAM of Version 1 that has a land/groove structure, the DVD-ROM, the DVD-R, the DVD-RW and the like that have a structure different from the land/groove structure, and the DVD-RAM of Version 2.0 and Version 2.1 that has the land/groove structure.
First, the “three-beam method” mainly adopted in detection of an error signal in the CD standard will be described. In an optical pickup apparatus, as shown in FIG. 26, a diffraction grating 320 for CD is arranged on an optical path between a semiconductor laser element 210 and a polarization beam splitter 230. The diffraction grating 320 for CD has a linear grating groove formed at an equal interval in a constant cycle and has a function of diffracting and branching a laser beam emitted from the semiconductor laser element 210 into at least three beams, that is, a main beam (0th-order light) and two sub-beams (±1st-order diffraction light flux).
These three beams pass through the polarization beam splitter 230, a collimating lens 240, and an objective lens 250 and then, as shown in the left side in FIG. 27, on a signal layer Da of an optical disc D, a main spot 100 corresponding to the main beam and sub-spots 101 and 102 corresponding to the two sub-beams, respectively, are formed. On the signal layer Da of the optical disc D, tracks D100 for recording the signal are provided periodically, and an interval δ between the main spot 100 and the sub-spots 101 and 102 in a disc radial direction is adjusted to match approximately ½ of a cycle Dtp of the track D100 by a means that rotationally adjusts the diffraction grating 320 for CD about the optical axis. Reflection light from the main spot 100 and the sub-spots 101 and 102 reaches the objective lens 250, the collimator lens 240, and the polarization beam splitter 230 again and a part of the light amount passes through the polarization beam splitter 230 and then, enters a photodetector 270 via a detection lens 260.
In the photodetector 270, as shown in the right in FIG. 27, light receiving surfaces 200a, 200b, and 200c corresponding to reflection lights of the main spot 100 and the sub-spots 101 and 102, respectively, are arranged. When the reflection lights of the main spot 100 and the sub-spots 101 and 102 are incident on the light receiving surfaces 200a, 200b, and 200c, respectively, a main-detection-light spot 200 corresponding to the main spot 100 and sub-detection-light spots 201 and 202 corresponding to the sub-spots 101 and 102 are formed, respectively.
Here, in a case where the main spot 100 is accurately scanning the track D100, the light amounts of the sub-detection-light spots 201 and 202 are the same. However, in a case where the scanning of the main spot 100 is deviated from the track D100, there will be a difference in the light amounts between the sub-detection-light spots 201 and 202. Then, by performing a subtraction process or the like on the light amounts of the sub-detection-light spots 201 and 202 using a subtractor 400 and the like, for example, a tracking error signal indicating scanning deviation in tracking is generated.
Next, the “inline type” mainly adopted for detection of an error signal in the DVD standard will be described. The inline-type optical system can basically detect a tracking error signal based on substantially the same optical system as the three-beam method. However, the inline type is different from the three-beam type optical system in that, as shown in the left in FIG. 29, a diffraction grating 340 for DVD in which the phase of a periodic structure of a grating groove formed in one half plane 341 is deviated by approximately 180 degrees from the phase of the periodic structure of the grating groove formed in the other half plane 342 is used.
Here, it is assumed that the diffraction grating 340 for DVD is provided at substantially the same position as that of the diffraction grating 320 for CD shown in FIG. 26 and is replaced with the diffraction grating 320 for CD. Also, in order to support to the inline method, it is assumed that arrangement positions of the diffraction grating 340 for DVD, the light collection optical system and the like are adjusted in such a manner that the main spot 100 and the sub-spots 101 and 102 formed on the signal layer Da of the optical disc D is formed on the same track D100 as shown in the left in FIG. 28.
When the light receiving surface 200a of the photodetector 270 is irradiated with the main beam for DVD that forms the main-detection-light spot 200, a subtractor 500a connected to the light receiving surface 200a calculates a difference in an output signal from the light receiving surface 200a and generates a main-push-pull signal Sa, for example.
When the light receiving surface 200b of the photodetector 270 is irradiated with a first sub-beam for DVD forming the sub-detection-light spot 201, a subtractor 500b connected to the light receiving surface 200b calculates a difference in an output signal from the light receiving surface 200b and generates a preceding sub-push-pull signal Sb, for example.
When the light receiving surface 200c of the photodetector 270 is irradiated with a second sub-beam for DVD forming the sub-detection-light spot 202, a subtractor 500c connected to the light receiving surface 200c calculates a difference in an output signal from the light receiving surface 200c and generates a lagging sub-push-pull signal Sc, for example.
As shown on the right in FIG. 28, the push-pull signal Sa detected from the main-detection-light spot 200 and the push-pull signals Sb and Sc detected from the sub-detection-light spots 201 and 202 corresponding to each of the sub-spots 101 and 102 are outputted in opposite phases to each other similarly to the three-beam method. After that, the push-pull signals Sb and Sc are added by an adder 510, and the added signal is subjected to a subtraction process by the subtractor 530 with respect to the push-pull signal Sa, whereby a tracking error signal in which respective offset components of the push-pull signals Sa, Sb and Sc are canceled out can be generated.
Recently, optical pickup apparatuses capable of recording and reproducing optical discs conforming to the CD standard as well as optical discs conforming to the DVD standard are being proposed. In such optical pickup apparatuses, in order to reduce costs through simplification of an optical system, a multi-laser unit is used that is equipped with a semiconductor laser element for CD that emits a first laser beam having a first wavelength of an infrared wavelength band 765 to 805 nm (nanometers) suitable for the CD standard and a semiconductor laser element for DVD that emits a second laser beam having a second wavelength of a red wavelength band of 645 to 675 nm suitable for the DVD standard.
Also, in such an optical pickup apparatus, in order to achieve further simplification of the optical system, a two-wavelength diffraction grating that supports both the three-beam type CD standard and the inline-type DVD standard is used (See patent Document 1 shown below, for example). For example, as shown in FIG. 29, a two-wavelength diffraction grating 300A is configured in such a manner that, with respect to two planes opposing in the thickness direction of an optical glass plate 360, the diffraction grating 320 for CD is fixed to one of the planes and the diffraction grating 340 for DVD is fixed to the other plane.
Other than the structure of the two-wavelength diffraction grating 300A shown in FIG. 29, a two-wavelength diffraction grating 300B of a structure as shown in FIG. 30, for example, is proposed (See Japanese Patent Laid-Open No. 2007-164962 (page 1, FIGS. 1 to 8), Japanese Patent Laid-Open No. 2007-149249 (page 1, FIGS. 1 to 7), for example). The two-wavelength diffraction grating 300B is configured in such a manner that the diffraction grating 320 for CD containing a liquid crystal material and the like is stacked on and fixed to the diffraction grating 340 for DVD and then, they are sandwiched and fixed between two optical glass plates 361 and 362, for example.
However, if the two-wavelength diffraction grating 300A or 300B in which the diffraction grating 320 for CD and the diffraction grating 340 for DVD as above are combined is used, when the first laser beam of the CD standard is made to be incident on the diffraction grating 320 for CD, for example, the first laser beam is diffracted by the diffraction grating 320 for CD and branched into three beams, that is, a main beam (0th-order light) and two sub-beams (±1st-order diffraction light fluxes). And the three beams are further diffracted and branched with the diffraction grating 340 for DVD.
As described above, the first laser beam or the second laser beam emitted from the multi-laser unit passes through both the diffraction grating 320 for CD and the diffraction grating 340 for DVD of the two-wavelength diffraction gratings 300A or 300B, and as a result, unnecessary diffracted light is generated since diffraction and branching are performed in the diffraction gratings 320 and 340 for CD and for DVD, respectively. As a result, there is a drawback that a detection accuracy of an error signal such as a tracking error signal is reduced.
Also, due to the generation of unnecessary diffracted light, transmittances of the 0th-order light and the ±1st-order diffraction light in the diffraction gratings 320 and 340 are lowered, and as a result, there is also a drawback that use efficiency of outgoing light emitted from the multi-laser unit is lowered.
Also, in order to be compatible with a plurality of types of optical discs D with different track pitches Dtp such as DVD-ROM, DVD-R, DVD-RW, DVD-RAM (Version 1, 2.0, 2.1) and the like without a malfunction, there is a market need for elaborate optical pickup apparatuses capable of providing easy control such as tracking control and the like and optical disc devices provided with the elaborate optical pickup apparatuses capable of providing easy control of tracking control and the like.
For example, there is a market need for an optical pickup apparatus in which an amplitude of an error signal such as a tracking error signal and the like due to displacement of the objective lens 250 is not decrease during data recording/reproducing of a plurality of types of optical discs D with different track pitches Dtp and an optical pickup apparatus in which offset does not remain in the error signal such as a tracking error signal and the like.
Further, there is a need for an inexpensive optical pickup apparatus and an inexpensive optical disc device which obviate the above drawbacks.